Problem 28 - General Chemistry

Question

Which of the following is most likely a tetrahedral complex? a. \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\) b. \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\right]\) C. \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\) d. \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{SO}_{4}\) e. \(\left[\mathrm{Co}(\mathrm{en})_{2}\left(\mathrm{H}_{2} \mathrm{O}\right) \mathrm{Cl}\right]^{2+}\)

Step-by-Step Solution

Answer

The tetrahedral complex is \([\mathrm{Zn}(\mathrm{NH}_3)_2 \mathrm{Cl}_2]\).

Step 1: Identify Tetrahedral Complexes Characteristics

Tetrahedral complexes usually have coordination number 4, meaning the central metal ion is surrounded by four ligands. They generally result in tetrahedral geometry rather than square planar due to steric and electronic reasons.

Step 2: Analyze Each Option

Review each complex for possibilities of coordination number and geometry.- (a) \([\mathrm{Zn}(\mathrm{NH}_3)_2 \mathrm{Cl}_2]\): Zn with coordination number 4, likely tetrahedral.- (b) \([\mathrm{Co}(\mathrm{NH}_3)_3 \mathrm{Cl}_3]\): Co has a coordination number of 6, more likely octahedral.- (c) \([\mathrm{Co}(\mathrm{NH}_3)_6]^{2+}\): Co has coordination number of 6, octahedral.- (d) \([\mathrm{Ni}(\mathrm{H}_2 \mathrm{O})_6] \mathrm{SO}_4\): Ni with coordination number 6, octahedral.- (e) \([\mathrm{Co}(\mathrm{en})_2(\mathrm{H}_2 \mathrm{O})\mathrm{Cl}]^{2+}\): Co has coordination number of 6.

Step 3: Assess Likelihood of Option (a) Being Tetrahedral

(a) \([\mathrm{Zn}(\mathrm{NH}_3)_2 \mathrm{Cl}_2]\) has Zinc at central, which often forms tetrahedral complexes in this ligand arrangement. Moreover, it has exactly 4 ligands which fits tetrahedral coordination number.

Key Concepts

Tetrahedral Complexes

Tetrahedral complexes are a fascinating subset of coordination compounds in chemistry. They are characterized by a central metal atom or ion that is surrounded by four ligands. This arrangement forms a geometrical shape reminiscent of a pyramid with a triangular base. Tetrahedral geometry often arises due to specific steric and electronic factors.

• A central metal with a coordination number of 4 is common in these complexes.
• The spatial arrangement minimizes ligand-ligand repulsions.
• Such a geometry is more likely when smaller ligands are involved, allowing for close packing around the central metal.

It is essential to note that not all complexes with four ligands are tetrahedral. The choice between tetrahedral and square planar geometry depends heavily on the metal and the type of ligands involved. Understanding these factors can assist you in predicting the correct geometry for a given complex.

Coordination Number

The coordination number is a key concept in coordination chemistry, representing the number of ligand atoms that are bonded to a central metal atom in a complex. This number is crucial for determining the geometry of a complex.

• A coordination number of 4 can lead to either tetrahedral or square planar geometry, although tetrahedral is more common.
• Coordination numbers of 6 often result in an octahedral geometry.
• The coordination number is determined by the bond-forming capacity of the ligands attached to the central metal.

In our exercise, identifying the coordination number helps in predicting the potential geometry. For example, those with higher coordination numbers like 6 tend to form octahedral structures, while a number of 4 suggests the possibility of a tetrahedral structure, especially when paired with transition metals.

Ligand Geometry

Ligands, which are molecules or ions that donate pairs of electrons to the central metal, significantly influence the overall geometry of a complex. They vary in size, charge, and the number of donation sites, which can markedly affect the metal center's geometry.

• Small ligands allow for closer proximity and often promote tetrahedral geometry.
• Larger or bulkier ligands might lead to different geometrical outcomes due to steric hindrance.
• Some ligands have multiple donor atoms, which can change the coordination number and, subsequently, the geometry.

Understanding ligand characteristics is crucial for predicting the overall structure of coordination complexes. For instance, ligands that bind strongly and compactly can push the structure into a tetrahedral formation due to their spatial distribution around the metal center.

Transition Metal Complexes

Transition metal complexes are a broad class of coordination compounds recognized for their diverse geometries and electronic properties. These complexes are formed when transition metals bind with ligands, allowing for a variety of configurations.

• These metals have partially filled d-orbitals, which participate in bond formation.
• The nature of the metal and its electronic configuration can dictate the geometry, ranging from tetrahedral to octahedral.
• Transition metals often display different oxidation states, which expand their chemical versatility and influence ligand interactions.

In the context of the exercise, recognizing the role of transition metals in influencing complex formation is key. Zinc, in particular, is a transition metal known for forming tetrahedral complexes, especially when partnered with ligands like NH\(_3\) and Cl\(_2\). Understanding these foundations allows for more accurate predictions of the structures and properties of various complexes.